The trend in the electronics industry today is toward higher circuit board densities creating a need for smaller and thinner integrated circuit packages. As the industry gets increasingly into such products as memory cards, smart cards, and Personal Computer Memory Card International Association (PCMCIA) cards, package thickness becomes a critical issue for these and other applications.
The semiconductor industry is presently producing one millimeter Quad Flat Pack (QFP) packages and the demand is growing for even thinner packages. It is becoming increasingly difficult to reduce package thickness substantially more using conventional plastic package structures. This is because plastic packages must inherently contain sufficient material to retain its structural rigidity.
FIG. 1 shows a prior art Quad Flat Pack plastic package 2 with lead frame 4, die 6, die attach pad 8, and wire bonds 10 that are encapsulated in a molded plastic 12 package. Conventional lead frames are typically 6 mils thick, although thinner lead frames have been used. FIG. 2 shows a lead frame assembly 20 prior to encapsulation where the die attach pad 8 is stamped from the same material as the lead frame 4 and supported by corner support arms attached to the lead frame not shown. The plastic material beneath the die attach pad 8 is at least approximately 6 mils thick which brings the thickness of the plastic base to in the range of about 18 to 22 mils. A thin package using this conventional plastic structure is generally about 40 mils thick (about 1 mm).
Another problem with the thicker or larger packages is that they impart larger residual stress on the die due to differing thermal expansion rates of the package material (usually plastic) and lead frame from that of the die. Another disadvantage is that larger packages are more likely to contain moisture after the preheat stage in the manufacturing process, which is not sufficient to drive off all the moisture, thus increasing the possibility of package cracking. What is needed is a thin package structure that avoids these problems and can be used in the applications mentioned above.